The prevalence of children with peanut allergy, the most common cause of life-threatening food allergic reactions, has doubled from 0.4% in 1997 to 0.8% in 2002. It persists for a lifetime in most subjects, yet little is known about peanut allergy in adults. While ~8% of US adults have a positive skin test to peanut allergen (PA), the majority tolerate peanuts and predictors of clinical reactivity are undefined. Thus, there is a need for biomarkers that identify adults at risk for PA-related anaphylaxis. Blood basophils bind IgE via FceRI and are effector cells of anaphylaxis. In food allergic subjects, both basophils and serum have unique characteristics. Basophil spontaneous histamine release (SHR, ranging from 25-100% of total cell content) is typically seen in those with food allergies, is modulated by food allergen exposure and can be transferred via serum to a healthy donor's basophils. Recently, allergen-induced basophil responses were correlated with the degree of clinical reactivity to milk allergen. Milk-induced basophil reactivity is greatest in children who are intolerant to both heat-denatured and fresh milk as compared to children who are intolerant only to fresh milk. In the latter group, reduced basophil reactivity is related to an allergen-specific, suppressive IgG. Wider diversity in serum IgE binding to PA-epitopes is found in subjects with severe PA-reactions. Our goal is to examine biomarkers that identify IgE-sensitized adults at risk for severe PA-reactions. The overall hypothesis is that subjects with severe PA-reactions have basophils with greater reactivity to PA, and serum that transfers SHR and has extensive diversity in IgE binding to PA-epitopes. We will compare the features of basophils and serum among 3 groups of peanut allergic adults. Group A are those with a history of severe peanut allergy (life-threatening reaction or high-risk due to severe asthma and thus cannot ethically undergo PA challenge), Group B are subjects with an intermediate history of peanut reactions confirmed by a double-blind, placebo-controlled food challenge (DBPCFC), and Group C are subjects with an intermediate history of peanut allergy who pass a DBPCFC. In Aim 1, we will test the hypothesis that subjects in group A have the greatest basophil SHR and reactivity as measured by dose-response curves for PA-induced histamine release and the induction of surface CD63 and CD203c. In Aim 2, we will test the hypothesis that serum from group A has greater ability to transfer SHR to healthy donor basophils than serum from groups B and C, and controls. We will also define the role of IgE in SHR transfer by selective IgE-depletion. In Aim 3, we will test the hypothesis that greater diversity in IgE-binding to PA-epitopes, detected by microarray, is related to the severity of peanut allergy. Also, we expect that serum IgG that competes with IgE-binding to arrays leads to reduced clinical severity and basophil PA-reactivity. The completion of these aims will be the most complete study of biomarkers to date in adults with peanut allergy. Food allergy is a health problem that affects 3-4% of adults and 6-8% of children in the United States. Our goal is to identify novel biomarkers that identify subjects at risk for severe food allergy reactions. We will compare the characteristics of basophils and serum factors (e.g., peanut specific IgE, IgG) among three groups of peanut allergic adults classified by the severity of their peanut-reactions.